TDC - Technical Questions & Answers - April 2005

Sport Truck, April 01, 2005

Filter, pump, or what?Q: I'm having fuel system problems with my '94 GMC Sierra pickup, which is equipped with the 5.7L V-8 and the automatic transmission. It seems to be starved for fuel. It hesitates or stalls completely. My friend Jim suggested that this era of GM trucks have had problems with the fuel filters, so I changed the filter. It was much easier than changing a fuel pump.

Changing the fuel filter didn't cure the problem. I checked to see if the fuel pump was pumping fuel, and it was. When I had the fuel filter off, I blew compressed air through the disconnected lines and nothing unusual came out. So, the fuel filter is new, the fuel pump works fine, and there aren't any restrictions in the fuel lines. I use additives and fuel-drying products during the winter months, so there shouldn't be any excess moisture in the tank.

Could my problem be something other than a fuel system one, or did I overlook something? I hope you can help me.Evan Mitchell,via e-mail

A: It sounds more like a fuel pump problem than a fuel filter issue. Just because a fuel pump moves fuel, doesn't mean it is doing it at the proper pressure. You should check or have it checked by a mechanic. The fuel pressure for your truck should be in the 9-13 psi range. If your readings are less that specified, driveability problems can be the result.

GM has a technical service bulletin (66-63-09) related to hard-starting or no starting problems. This service bulletin discusses the possibility of a ruptured fuel pump pulsator. That can lead to decreased or no fuel pressure. Either case could cause the symptoms that you have described. The fuel pump pulsator is a flexible connector that is located between the pump and the metal fuel line in the gas tank.

You mentioned that you use wintertime chemicals (such as ethanol) in your fuel system. While they may dry the fuel, they can also damage the flexible part of the fuel pump pulsator. The pulsator can also deteriorate with age. The GM recommended replacement fuel pump pulsator is made with Viton, which is more resistant to the additives.